A. Field of Invention
The present invention relates generally to the electroplating of alloys of two metals having reversible potentials sufficiently separated that they cannot be readily co-deposited from their simple cations. Specifically, a copper-rich alloy is electrodeposited from a plating solution in which, at typical current plating densities, the potential is positive to the reversible plating potential of the minor component to be alloyed with the copper. This minor component is a metal less noble than copper and is incorporated in the alloy by the mechanism of underpotential deposition (UPD) on the co-deposited copper. More particularly, the present invention is related to a methane sulfonic acid solution, which contains the cations of copper and another, less noble metal, useful in the deposition of copper-rich alloys by facilitating the UPD of the less noble metal on copper. The occurrence of UPD during electroplating results in the deposition of an alloy of copper with the less noble metal.
B. Description of Related Art
Electrodeposition of copper deposits containing small amounts of alloying metals is useful in a number of applications in which it is desirable to modify the physical or chemical properties such as hardness and corrosion resistance of the copper. Copper alloys are also being investigated, for their resistance to electromigration, as materials for chip wiring. For this application, it is important that the alloying material be at a very low level and have a very small effect on the copper resistivity. Electromigration-resistant copper alloys meeting these criteria are, however, difficult to deposit by the usual methods.
Certain alloys, including commercially important copper-based alloys, are difficult to electrodeposit because the components of the alloy have widely different reversible deposition potentials. The standard electrode potentials of copper (0.34 volts) and tin (-0.14 volts) in divalent salt solutions of the simple ions, for example, are about 0.5 volts apart. CRC Handbook of Physics and Chemistry, 67th ed., pp. D-151-154 (CRC Press, Boca Raton 1986). Thus, co-deposition of the two metals in a coherent form from mixtures of their simple salts is difficult, especially when copper is the major component of the alloy.
Conventional processes typically overcome such difficulties either by complexing the ion of the more noble metal, copper, in solution, to bring its reversible potential closer to that of the less noble metal ion (e.g., tin), or by depositing the more noble metal at its limiting current.
Neither of these expedients is entirely satisfactory. First, one of the most effective complexants, cyanide, poses environmental hazards. Second, complexation generally requires higher pH values because, at lower pH, common complexing agents are usually protonated and, consequently, do not function effectively. Use of complexants and higher pH complicates the solution chemistry and may reduce the current efficiency of electrodeposition and change the properties of the electrodeposit. High pH solutions may also be incompatible with polymers used in microelectronic fabrication. Third, deposition of the major component of the alloy at its limiting current is highly undesirable, because such deposits generally have poor metallurgical properties. In general, deposition at the diffusion limited current can be successfully used only for the minor component of an alloy. Thus, the minor component must be the more noble metal for this mechanism of codeposition to be feasible.
Conventional methods for electrodepositing copper and copper-based alloys have been used for many years. Bronze, which is a copper-tin alloy, was plated over 100 years ago using a bath containing copper as a cyanide complex and potassium stannate. A. Brenner, Electrodeposition of Alloys, ch. 15 (Academic Press, New York, 1963). Improvements were made to these plating processes by establishing the optimum operating conditions of the processes and by incorporating additives into the plating baths. Brighteners for producing a brilliant copper finish, levelers to impart a smooth finish, wetting agents, and agents to promote anode corrosion are among the prior art additives that have been developed and described.
Some of the patents that generally disclose these prior art electroplating processes and the additives used include:
U.S. Pat. No. 2,910,413 is one of the many patents related to addition agents for copper plating. The '413 patent discloses a brightening additive that may be used in conjunction with other organic addition agents and that is intended to be useful in electroplating copper, bronze, and brass. The brightener works best in conjunction with a sulfonic acid containing a halogen or pseudo-halogen which is at low concentration and is not a major solution component.
U.S. Pat. No. 3,023,150 is another of the numerous patents in the field of brightening agents for acid copper plating. The '150 patent generally discloses additives that are intended to be useful in all types of plating systems including electrodepositing of copper, brass, and bronze. Among the brighteners specified are derivatives of mercaptopropane sulfonic acid which serve as addition agents at low concentrations.
U.S. Pat. No. 4,038,161 does not teach alloy plating but discloses chemical addition agents to provide levelling behavior in acid copper plating.
U.S. Pat. No. 4,347,107 pertains primarily to the deposition of tin. Although the '107 patent suggests that copper and rhodium be incorporated in the tin deposit, a tin-based alloy is electrodeposited. The copper must be at a very low concentration in the solution and is deposited at its diffusion limiting current because it is the more noble of the two metals in the alloy. Good quality copper-based alloys cannot be deposited near the diffusion limiting current and, therefore, the teaching of the '107 patent could not be used to deposit copper-rich alloys.
U.S. Pat. No. 4,381,228 also teaches a method of depositing tin and tin-rich alloys of copper and rhodium using a mixed fluoroboric/sulfuric acid system to avoid the problem of anode passivation. Appropriate addition agents are also used.
U.S. Pat. No. 4,389,286 teaches deposition of high-copper alloys of copper-tin and copper-lead using a glucoheptonic acid salt as a complexing agent instead of cyanide. Although the solution does not contain cyanide, it is still highly alkaline, with a pH above 12, rendering the solution incompatible with most microelectronic processes because polymers are generally attacked at very high pH.
U.S. Pat. No. 5,039,576 teaches an electroplating bath, cell, and method for the electrodeposition of tin-bismuth alloys onto a conductive substrate using an alkyl sulfonic or polysulfonic acid or salt as the electrolyte. Although copper plating using methane sulfonic acid is disclosed, neither copper alloys nor UPD is mentioned.
The prior art plating processes have many drawbacks. First, the solutions used to electroplate both copper and copper-based alloys generally include additives. Solutions used to electroplate copper-rich alloys typically use toxic chemicals, such as cyanide, which present safety, handling, and disposal problems. In the absence of a complexant like cyanide, the less noble metal will not begin to deposit unless the plating current density is higher than the diffusion limited current density for copper. For copper-rich alloys, the result of plating the alloy above the copper limiting current will be a deposit with poor physical and metallurgical properties. Finally, the prior art processes are often incompatible with materials such as polymers used in microelectronic processing.
The phenomenon of deposition from solution of atomic layers of a metal on a foreign metal substrate at potentials positive to its reversible Nernst potential is known as underpotential deposition (UPD). The name derives from the fact that the metal monolayer is formed before bulk deposition can occur. Typically, a less noble metal may form a UPD layer on a more noble metal.
The thermodynamic equilibrium potential for UPD is determined by: 1) the activities of the ions in solution, 2) the activity of the solid phase (which is not unity in the case of monolayers), and 3) the specific interactions between the deposit and the substrate. Thus, the UPD phenomenon depends on the substrate-deposit pair chosen. The degree of interaction is related to the difference between the work functions of the two metals. Examples of widely studied UPD systems are Pb, Sn, Cd, Ag, Cu (and others) on Au; Pb, Sn, Tl, As, Cu (and others) on Ag; and Pb, Zn, Cd (and others) on Cu. An extensive list would include many other metal pairs.
An advantage of exploiting UPD in the deposition of alloys is that the very metal pairs that give strong UPD interactions, a less noble metal on a more noble metal substrate, are the most difficult pairs to co-deposit using conventional electrodeposition technology. Thus, UPD makes it possible to electrodeposit alloys that are difficult to deposit by other means.
Although UPD layers, which are on the order of a monolayer in thickness, usually form and desorb nearly reversibly, the formation of surface alloys between a UPD metal layer and the substrate metal has been observed in the prior art. D. Kolb, "Physical and Electrochemical Properties of Metal Monolayers on Metallic Substrates," in Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, p. 125, H. Gerischer and C. Tobias, eds. (Wiley Interscience, New York, 1978). Such surface alloy formation was studied under near-equilibrium conditions. The substrate was a well-defined surface of the more noble metal, and the solution contained only cations of the less noble metal in an inert electrolyte. The role of UPD in the non-equilibrium phenomenon of continuous co-deposition of the two metals has not been studied.
For alloy deposition to occur by UPD during deposition of a more noble metal from a solution also containing ions of a second, less noble metal, three conditions must be met: 1) the deposition reaction must take place in the potential region of UPD, 2) the UPD layer of the less noble metal must be incorporated in the alloy rather than desorbing, and 3) the solution must not contain any species which are strongly absorbed on the surface and inhibit the formation of UPD layers.
The present invention incorporates UPD to overcome the shortcomings of existing processes used to electrodeposit copper-based alloys. Specifically, a new electroplating solution and a process of using that solution are provided. An object of this invention is to exploit UPD to deposit copper-rich alloys having components with very different reversible electrochemical potentials, with Cu--Sn and Cu--Pb being exemplary alloys to be plated.
Another object is to provide a non-complexing acid electrolyte solution and method of using that solution for plating copper-rich alloys in which the deposition of neither component is diffusion controlled (i.e., deposited at its limiting current). Yet another object is to provide a solution for plating such alloys which does not require additives, does not contain complexants for ions of either of the metal alloying components, and does not include toxic chemicals such as cyanides.
It is still another object of the present invention to provide a solution and method of using that solution for plating copper-rich alloys that are compatible with materials, such as polymers, used in microelectronic processes. It is a further object to assure good quality metallurgical properties of such alloys.